Add qrpn module

modules/qrpn/qrpn.h

@@ -0,0 +1,34 @@
+#include <stdint.h>
+#include <stddef.h>
+#include <complex.h>
+#include <stdio.h>
+
+#define BASEUNITS 7
+
+struct quantity {
+    /* this is the only time you will ever catch me using double complex */
+    double complex value;
+    int8_t units[BASEUNITS]; /* metre, kilogram, second, ampere, kelvin, candela, mol */
+    uint8_t flags;
+};
+
+#define QRPN_NOERR 0
+#define QRPN_ERROR_NOT_ENOUGH_STACK 1
+#define QRPN_ERROR_INCONSISTENT_UNITS 2
+#define QRPN_ERROR_MUST_BE_INTEGER 3
+#define QRPN_ERROR_TOKEN_UNRECOGNIZED 4
+#define QRPN_ERROR_RATIONAL_NOT_IMPLEMENTED 5
+#define QRPN_ERROR_MUST_BE_UNITLESS 6
+#define QRPN_ERROR_DOMAIN 7
+#define QRPN_ERROR_DIMENSION_OVERFLOW 8
+#define QRPN_WAS_A_UNIT 9
+
+#define FLAG_UNIT_ENTERS_AS_OPERAND 1
+#define FLAG_SI_BASE_UNIT 4
+#define FLAG_SI_DERIVED_UNIT 8
+
+int qrpn_evaluate_token(struct quantity ** stack_p, size_t * S_p, const char * const token);
+char * qrpn_error_string(const int status);
+void fprintf_stack(FILE * fh, struct quantity * stack, const size_t S);
+void fprintf_quantity(FILE * fh, const struct quantity quantity);
+int qrpn_try_token(const struct quantity * const stack, const size_t S, const char * const token);